1. Field of the Invention
This invention relates to seismic surveying, and is more particularly concerned with the use of seismic surveying for monitoring hydrocarbon reservoirs beneath the seabed.
2. Description of the Related Art
In reservoir monitoring, two or more sets of seismic data signals are obtained from the subsurface area containing the reservoir by conducting two or more seismic surveys over the area at different times, typically with time lapses between the seismic surveys varying between a few months and a few years. The acquisition and processing of time-lapsed three dimensional seismic data signals over a particular subsurface area (commonly referred to in the industry as “4D” seismic data) has emerged in recent years as an important new seismic surveying methodology.
The purpose of 4D seismic surveys is to monitor changes in the seismic data signals that can be related to detectable changes in geologic parameters. These (not necessarily independent) geologic parameters include fluid fill, propagation velocities, porosity, density, pressure, temperature, settlement of the overburden, etc. Of primary interest are changes taking place in the reservoir itself. Analysing these changes together with petroleum production data assists the interpreter in understanding the complex fluid mechanics of the system of migration paths, traps, draining or sealing faults making up the reservoir. This provides information regarding how best to proceed with the exploitation of the reservoir, for example where to place new production wells to reach bypassed pay zones and where to place injectors for enhanced oil recovery, and so helps to produce an increased quantity of hydrocarbons from the reservoir, often in a more cost effective way.
An important precondition to being able to map detectable changes of geological parameters is that the sets of seismic data signals which have been acquired at different times must be calibrated so they match each other. The phrase “match each other” in this context means that images of the seismic data signals reflected from places where no geological parameter changes have taken place must appear substantially identical in the different seismic data signal sets. This requires a high degree of repeatability between the successive surveys.
In one prior art method of reservoir monitoring, trenches are formed in the seabed above the reservoir, and seismic cables each having a plurality of seismic sensors distributed along its length are permanently installed in the trenches. A seismic survey of the reservoir is then periodically performed, eg at six or twelve month intervals, by connecting the cables to a seismic survey vessel, typically using a remotely operated vehicle (ROV) on the seabed to effect the connection, and then repeatedly operating a seismic source, eg an airgun array, which is towed by another vessel back and forth in the water above the reservoir.
This prior art method of reservoir monitoring provides quite a high degree of repeatability, but suffers from the disadvantage that the cost of the installed cables is very high, typically several tens of millions of dollars, in relation to the amount of use obtained from them. It is an object of the present invention to alleviate this disadvantage.